The primogeniture of inheritance refers to the first-born son on the side of the father by any of his wives (if he lived in polygamy). The law of such primogeniture is found in Deut. xxi. 16 et seq., according to which the first-born is to receive a double portion of the inheritance. The passage referred to, however, did not introduce this right, for the preference of the first-born, as the issue of the "first strength" () of the father, existed in patriarchal times (comp. Gen. xxv. 31, xxvii. 29, xlviii. 13, xlix. 3). It is generally assumed that the prerogatives of the first-born consisted in a kind of potestas over the family; in a double share of inheritance (comp. I Chron. v. 1); and in the right to the priesthood (comp. Targ. Onḳ. and Yer. to Gen. xlix. 3). From Gen. xxv. 31 (comp. xxvii. 36) it appears also that God's promises to the Patriarchs were considered as attached to the line of the first-born. But, as the cases of Esau and Reuben (and Ishmael, Gen. xxi.) show, it was possible for the father to deprive the first-born of his right; and the lawgiver in Deuteronomy prohibits the misuse of parental power in favor of a younger son by a favorite wife. In the succession to the throne primogeniture was generally taken into consideration (comp. II Chron. xxi. 3), though it was not always decisive, as appears in the case of Solomon (I Kings i. 30, ii. 22) and of Abijah (II Chron. xi. 22; and comp. Junior Right).

Rabbinical law further specifies and qualifies the right of primogeniture. Only the first-born—not the eldest surviving son who has been preceded by another child that has died—and only such a one as, by a normal birth and not by a surgical operation, came into the world in the lifetime of his father is entitled to the double share (Bek. 46a, 47b; B. B. 142b). Furthermore, the first-born of a first-born does not receive a double portion of the inheritance of the grandfather who dies before the father (Bek. 51b; B. B. 124a). On the other hand, if the first-born dies before his father his right passes over to his children, even to daughters (B. B. 122b). Neither the inheritance left by the mother nor posthumous improvements () of and accessions () to the inheritance left by the father are subject to the right of primogeniture (Bek. 51a; B. B. 122b, 124a). The double share of the first-born is not one-half of the property, but double the share of each of the other brothers. If there are, for instance, four brothers, the property is divided into five parts, the first-born receiving two-fifths and the others each one-fifth. But the portion of the first-born is affected by either the death or the birth of another brother after the demise of the father (B. B. 123a, 142b). As the double share of the inheritance entails a double share in the obligations on the part of the first-born, both may be waived by him (B. B. 124a).

It is apparent from the preceding regulations that both in the Old Testament and in the rabbinical law the prerogative of primogeniture was not conceived as an inalienable right inherent in the first-born, but rather as a gift by the Law, prompted by economic considerations. The eldest son, who was to take the father's position, was to be placed economically in a condition to be able to preside with dignity over the family—something like the right of majorat. It is, moreover, probable that the first-born had the obligation of maintaining the female members of the family who remained in the household. For the Talmudic regulation of the status and maintenance of the unmarried daughters after the father's death see Ket. 68a, b.

The primogeniture of redemption refers to the male first-born on the mother's side and applies to both man and beast: "Sanctify unto me all the first-born, whatsoever openeth the womb among the children of Israel, both of man and beast: it is mine"(Ex. xiii. 2). In the manner of the sanctification of these first-born the following distinctions are drawn:

1. The first-born of a clean animal had to be brought to the sanctuary within a year from the eighth day of its birth (Ex. xxii. 30). If without a blemish it was treated as a sacrifice; i.e., the blood was sprinkled and the fat burned on the altar. As regards the disposal of the flesh there is a difference between the laws in Deuteronomy and those in Numbers. According to the former (Deut. xv. 19 et seq.; comp. xii. 6 et seq., 17 et seq.; xiv. 23) the flesh is eaten by the owner in a sacrificial meal, like that of the "shelamim," while according to the latter (Num. xviii. 17 et seq.; comp. Ex. xxii. 29) it fell to the priest. The latter practise prevailed in the time of Nehemiah (Neh. x. 37) and Josephus (Josephus, "Ant." iv. 4, § 4). Had the animal a blemish, it was treated like any other common food (Deut. xv. 21-23).

2. The first-born of an unclean animal had to be redeemed, when a month old, according to the estimation of the priest, with the addition of one-fifth (Lev. xxvii. 27; Num. xviii. 15 et seq.). The first-born of an ass was either ransomed by a sheep or killed, its neck being broken (Ex. xiii. 13, xxxiv. 20). In Josephus' time (l.c.) all unclean animals were redeemed with one and a half shekels.

In the Talmud the fact that the first-born in this case must be a is emphasized. Thus a first-born son whose birth has been preceded by a miscarriage, or by a still-birth, or by the birth of a monstrosity, or one who was himself brought forth by a surgical operation, is not due to the priesthood. On the other hand, if two wives of the same man both bear sons as first-born children, each must be redeemed (Bek. viii. 1, 2, 46a, 47b).

In Ex. xiii. 11-15 and Num. iii. 12 et seq. (comp. ib. 40 et seq. and viii. 15-18) the dedication of the first-born to Yhwh is connected with the slaying of the first-born of Egypt and the consecration of the Levites to the service of the sanctuary. By destroying the first-born of Egypt and sparing those of Israel, Yhwh acquired an especial ownership over the latter. But as it was not feasible to select the first-born of the entire nation and thus disturb the family organization, the Levites were substituted for them; and, indeed, rabbinical tradition assigns the priesthood to the first-born until the completion of the Tabernacle (Zeb. 112b, 115b; comp. Targ. to Ex. xxiv. 5 and Rashi and Ibn Ezra to Ex. xix. 22, 24). The view implied in the passages quoted seems to be that the Levites took the place of only those first-born which Yhwh actually spared in Egypt, and that while the Levites continued to serve at the sanctuary, all the first-born after the Exodus were nevertheless the property of Yhwh, and therefore had to be redeemed, just as the 273 first-born who surpassed the number of the Levites at Sinai had to be redeemed each with five shekels (Num. iii. 45-51). Doubtless there is here also the adaptation of an ancient custom (comp. Gen. iv. 4). The dedication of the first-born of man is the extension and application by analogy of the custom of consecrating to God the first-fruits of the soil and the firstlings of animals (comp. Ex. xxii. 28 et seq.), a custom found also among other peoples. In Israel this dedication had the significance of an acknowledgment that it was Yhwh's "heritage," that it owed to Him all which it had and was.

The interpretation of the custom of redeeming the first-born as a modification of an older custom of sacrificing the first-born sons in connection with the Passover feast (Baudissin, in Herzog-Plitt, "Real-Encyc." 2d ed., x. 176; comp. also Frazer, "The Golden Bough," 2d ed., ii. 48), has no foundation in history. There are instances in later times attesting not only the custom of sacrificing children, but also the fact that at times the first-born was preferred as a victim (II Kings iii. 27; Micah vi. 7; Ezek. xx. 26); but there is nowhere a trace of the demand of such a "blood-tax" on the part of the Deity or Lawgiver from the people, and its existence is unknown even among the Canaanites (comp. Wellhausen, "Prolegomena," 2d ed., p. 91; Robertson Smith, "Religion of the Semites," 2d ed., p. 464; and Toy on Ezek. xx. 26 in "S. B. O. T.").

Since the destruction of the Temple and cessation of sacrifices the dedication of the first-born of clean animals is limited to their being kept inviolate and exempt from any use (comp. Deut. xv. 19), unless they have or receive some blemish, in which case they may be slaughtered for food. The redemption of the first-born of an ass and of man is still carried out according to the Biblical ordinances, and the redemption of the first-born son () is a festive occasion. From such redemption are exempt not only priests and Levites, but also their children (Bek. 4a, 47a). Adult first-born on either side are also obliged to fast on the eve of Passover, unless they are released from the obligation by some festive celebration, such as the completion of the study of a tract of the Talmud ("siyyum"; comp. "Yad," Bekorot, xi. 17; Yoreh De'ah, §§ 300, 305, 321).